The disclosure relates generally to ion exchange membranes, and more particularly to ion exchange membranes for use in electrochemical fuel cells, such as PEM fuel cells. More particularly still, the disclosure relates to such ion exchange membranes having increased durability.
A PEM fuel cell employs a membrane electrode assembly (MEA) in which the membrane is a proton exchange membrane, or polymer electrolyte membrane, (PEM). The membrane is disposed between anode and cathode electrodes respectively. The catalyzed cathode and anode serve to induce the desired electrochemical reactions. In addition to the aforementioned elements which comprise the membrane electrode assembly, there may be gas diffusion layers positioned outside of the electrodes. Cumulatively, these various elements comprise a unitized electrode assembly (UEA).
Reactants, typically an oxidant such as oxygen or air and a fuel such as hydrogen, are flowed over respectively opposite sides of the membrane to obtain the requisite electrochemical reaction. A seal about the perimeter of the membrane electrode assembly or the unitized electrode assembly normally serves to keep the reactants separate. This seal creates a non-active region portion to the membrane with respect to the desired electrochemical reaction.
The ion exchange membranes typically used in a PEM fuel cell have been polymer electrolyte membranes having cation exchange groups, and have included hydrocarbon-based membranes or those prepared from flouropolymers, and which contain sulfonic acid functional groups. A representative perflourosulfonic acid/PTFE copolymer membrane is available from DuPont Inc. under the trade name Nafion®.
From the standpoint of financial cost and system reliability, the durability and operational lifetime of a fuel cell are important. Unfortunately, failure modes may exist which have an adverse impact. One such mode involves the degradation of the membrane. This matter is discussed in PCT Application PCT/US2004/044013 having International Publication Number WO 2006/071234, which describes how oxygen may diffuse from the cathode to the anode through the membrane and can form peroxide by reacting with hydrogen at low potential at the anode catalyst surface. The peroxide can dissociate into highly reactive free radicals, which in turn may rapidly degrade the membrane. That published application describes an arrangement for extending not only the membrane, but also the catalyzed layers and possibly the electrodes into the non-active region associated with the edge seal. In this way, oxygen and/or hydrogen and any resulting peroxide which diffuses into the edge seal area are consumed by the catalyzed layers to prevent decomposition of the membrane.
U.S. application Ser. No. 10/738,962, published as U.S. Patent Application Publication 2005/0136308, describes the application of an additive non-uniformly to the MEA to address much the same problem. The additive is selected from “a radical scavenger, a membrane cross-linker, a hydrogen peroxide decomposition catalyst and a hydrogen peroxide stabilizer”. A number of examples are provided of additives that may serve to provide at least one of the aforementioned four functions. The principal focus of these additives is to interact with hydrogen peroxide in a manner that reduces the adverse nature of the peroxide itself. It suggests that the additive(s) be located in regions of the membrane subject to greatest potential chemical degradation.
While some advantages may be realized through the use of one or both of the aforementioned arrangements, they nevertheless remain deficient with respect to long term stability and structural integrity, or durability. One particular adverse characteristic is membrane degradation, which may be monitored by measuring one or more parameters, such as the rate of fluoride release in the instance of a fluoropolymer-based membrane. Generally speaking, the greater the rate of fluoride release from such a membrane, the greater the rate of membrane degradation. This release of fluoride may be a function of the formulation of the membrane material and is, for a given material formulation, strongly dependant on the operating conditions of the fuel cell in which the membrane is used. It has been observed that under accelerated load cycling conditions, membrane degradation is more severe in the active area-seal edge region compared to the rest of the active area. Although the reasons for such degradation are not fully understood, they perhaps include local conditions of heat and/or dryness. Still further, most of the prior art addresses only one of the components of membrane degradation—either mechanical or chemical—and most offer solutions that target solely chemical causes of membrane degradation.